The production of liquid crystal displays such as, for example, active matrix liquid crystal display devices (AMLCDs) is very complex, and the properties of the substrate glass are extremely important. First and foremost, the glass substrates used in the production of AMLCD devices need to have their physical dimensions tightly controlled.
In the liquid crystal display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred because of their ability to transport electrons more effectively. Poly-crystalline based silicon transistors (p-Si) are characterized as having a higher mobility than those based on amorphous-silicon based transistors (a-Si). This allows the manufacture of smaller and faster transistors, which ultimately produces brighter and faster displays.
One problem with p-Si based transistors is that their manufacture requires higher process temperatures than those employed in the manufacture of a-Si transistors. These temperatures range from 450° C. to 600° C. compared to the 350° C. peak temperatures employed in the manufacture of a-Si transistors. At these temperatures, most AMLCD glass substrates undergo a process known as compaction. Compaction, also referred to as thermal instability or dimensional change, is an irreversible dimensional change (shrinkage) in the glass substrate due to changes in the glass' fictive temperature. “Fictive temperature” is a concept used to indicate the structural state of a glass. Glass that is cooled quickly from a high temperature is said to have a higher fictive temperature because of the “frozen in” higher temperature structure. Glass that is cooled more slowly, or that is annealed by holding for a time near its annealing point, is said to have a lower fictive temperature.
The magnitude of compaction depends both on the process by which a glass is made and the viscoelastic properties of the glass. In the float process for producing sheet products from molten glass, the glass sheet is cooled relatively slowly from the melt and, thus, “freezes in” a comparatively low temperature structure into the glass. The fusion process, by contrast, results in very rapid quenching of the glass sheet from the melt, and freezes in a comparatively high temperature structure. As a result, a glass produced by the float process may undergo less compaction when compared to glass produced by the fusion process, since the driving force for compaction is the difference between the fictive temperature and the process temperature experienced by the glass during compaction.
Specifically, when a glass is held at an elevated temperature, its structure relaxes towards the heat treatment temperature. Because a glass substrate's fictive temperature is almost always above the relevant heat treatment temperatures in thin film transistor (TFT) processes, this structural relaxation causes a decrease in fictive temperature which therefore causes the glass to compact (shrink/densify). Such compaction is undesirable because it creates possible alignment issues during the display manufacturing process which in turn results in resolution problems in the finished display.
There are several approaches to minimize compaction in glass. One is to thermally pre-treat the glass to create a fictive temperature similar to the one the glass will experience during the p-Si TFT manufacture. There are several difficulties with this approach. First, the multiple heating steps employed during the p-Si TFT manufacturing process create slightly different fictive temperatures in the glass that cannot be fully compensated for by a pretreatment. Second, the thermal stability of the glass becomes closely linked to the details of the p-Si TFT manufacturing process which is to be used, which could mean different pretreatments for different end-users. Finally, pretreatment adds to processing costs and complexity.
Another approach is to slow the rate of strain at the process temperature by increasing the viscosity of the glass. This can be accomplished by raising the viscosity of the glass. For example, the annealing point of a glass represents the temperature corresponding to a fixed viscosity for a glass, i.e., 1013.2 poise, and thus an increase in annealing point equates to an increase in viscosity at fixed temperature. The challenge with this approach, however, is the production of high annealing point glass that is cost effective. Higher anneal point glasses typically employ higher operational temperatures during their manufacture thereby reducing the lifetime of the fixed assets associated with glass manufacture.
Moreover, a high annealing point is just one of a number of properties that are desirable for a glass composition that is to be used to produce display substrates. Other desirable properties include a high strain point, a low density, and a CTE (coefficient of thermal expansion) compatible with silicon-based electronic components.
A particularly important property is the ability of display manufacturers to cut sheets made from the glass composition into smaller pieces without the generation of large amounts of glass chips and/or the creation of excessive cracking, e.g., lateral cracking, at the cut lines, i.e., without substantial damage to the glass sheets as a result of the cutting. During the etching that takes place during TFT production, defects at the edges of a glass substrate, such as lateral cracks, tend to increase in size. As a consequence, electronic components, such as display drivers, need to be moved inboard from the edges of the substrate, thus increasing the size of the bezel needed to hide those components in a finished display. Such large bezels are incompatible with modern display design which favors small bezels. For ease of reference, the word “cutable” will be used herein to describe (1) glass sheets that can be cut into smaller pieces without the generation of excessive amounts of glass chips and/or lateral cracks, and (2) the glass compositions from which such glass sheets are produced.